1. Field of the Invention
This invention relates to recovery of hydrogen pyrolyzed hydrocarbon gases and/or refinery off-gases and especially relates to recovery of hydrogen by absorption with a preferential physical solvent from de-acidified, compressed, and dried hydrocarbon gases containing olefins.
2. Review of the Prior Art
Hydrogen is present in large quantities in thermally or catalytically cracked gas streams or in refinery off-gases and is commonly associated with olefins, such as ethylene and propylene. In addition to hydrogen, these gases generally comprise methane, carbon monoxide, carbon dioxide, acetylene, ethane, methyl acetylene, propadiene, propylene, propane, butadienes, butenes, butanes, C.sub.5 's, C.sub.6 -C.sub.8 non-aromatics, benzene, toluene, xylenes, ethyl benzene, styrene, C.sub.9 -400.degree. F. gasoline, 400+.degree. F. fuel oil, and water.
Numerous processes are known in the solvent absorption art for isolation and recovery of olefins from cracked, refinery, and synthetic gases containing these unsaturated compounds. Some processes utilize specific paraffinic compounds as an absorption oil, and others utilize an aromatic absorption oil as a solvent within an absorber column or an absorber-stripper column having a reboiler.
Thermal cracking of hydrocarbon feedstocks in pyrolysis furnaces for production of ethylene has been an established technology since the 1940's. The pyrolysis furnace gases were sent to the recovery section of an ethylene plant in which the first fractionation column was a front-end demethanizer operating at about -150.degree. C. The de-acidified, compressed, dried, and chilled pyrolysis gases were fed to the demethanizer after five compression stages to 500 psia. The demethanizer bottoms were fed to a deethanizer, and the demethanizer overhead, rich in hydrogen, was fed to a cryogenic unit which recovered additional ethylene from the fuel gas stream. A back-end acetylene removal system, such as a series of two acetylene reactors, was typically located between the deethanizer and the C2 splitter or between the depropanizer and the C3 splitter. This arrangement caused the production of large amounts of green oil, a polymer formed from olefins and diolefins, which was likely to freeze in the C2 splitter or accumulate in the ethane vaporizer.
The parent application avoids this problem by processing overhead gases from a heatpumped deethanizer or depropanizer, which is coupled with a front-end catalytic acetylene hydrogenation reactor, in an absorber-stripper configuration capable of recovering 75% to 95% of contained ethylene from the reactor effluent gases and subsequently processing the overhead gases from the absorber-stripper column to recover the contained solvent and remaining 5% to 25% of ethylene in a tail-end demethanizer.
The process of the parent application can be applied to all feedstocks for any conventional ethylene plant, but it is exemplified in the application by using a full range naphtha feedstock for a plant with a front-end depropanizer.
In this plant, the naphtha feedstock is vaporized and sent to the pyrolysis furnaces, and the furnace effluent is indirectly quenched in transfer-line exchangers before direct quench in the oil quench tower. Fuel oil fractions are produced from the quench system. Heat recovery from the hot furnace effluent is accomplished in the oil quench system by heat exchange with other process loads and generation of dilution steam.
The oil quench tower overhead is cooled further in the water quench system where the dilution steam is condensed. Heat is recovered from the circulating quench water by heat exchange with other process loads, especially the regeneration column feed preheating load so that there is an energy synergism within the overall system.
The cooled water quench tower overhead is compressed in three stages to the operating pressure of the front-end deethanizer/depropanizer. At the cracked gas compressor third stage discharge, acid gases are removed by a combination of amine and/or caustic systems. The acid gas-free cracked gas is then dried before entering the fractionation section of the plant.
A low-pressure debutanizing stripper is located in the compression train to remove C5 and heavier fractions from the cracked gas. No high-pressure stripper is required in the compression train.
A front-end heat pumped deethanizer/depropanizer system is coupled with a front-end selective catalytic acetylene hydrogenation reactor system. The front-end heat pumped depropanizer permits fractionation at low pressure and condensation at high pressure. Fouling is minimized when the depropanizer is operated at low pressure.
The energy for heat pumping of the deethanizer/depropanizer is provided by the fourth stage of the cracked gas compressor. At the compressor discharge, acetylene is selectively hydrogenated to ethylene in the front-end reactor system. In addition, heaver C.sub.3 and C.sub.4 acetylenes and diolefins contained in the depropanizer overhead are selectively hydrogenated to their respective olefins, resulting in overall olefin gains across the reactor system. No green oil is formed across this reactor system.
The acetylene-free C3-and-lighter portion of the cracked gas leaves the reactor and is dried in a dehydrator to remove trace quantities of moisture. This C3-and-lighter fraction leaves the depropanizer reflux drum and enters the solvent extraction system for recovery of C2-plus hydrocarbons.
The C3-and-lighter fraction is fed to the absorber column. The C2's and C3's are absorbed by the solvent while methane and lighter components, together with some ethylene, leave the top of the absorber. This overhead stream is fed to a small tail-end demethanizer where essentially all the C2's are recovered. Additionally, any solvent present in the absorber overhead is recovered and returned to the absorber. The demethanizer is auto refrigerated by means of turbo expanders. No external refrigeration is required for the tail-end demethanizer.
The rich solvent from the bottom of the absorber is fed to a solvent regenerator where the demethanized C2's and C3's are recovered as overhead product. The lean solvent is returned to the absorber after heat recovery.
The C2's and C3's are further separated in a conventional deethanizer to produce a C2 and a C3 fraction. These two fractions are then processed in their respective superfractionators to produce polymer grade ethylene and propylene products. Ethane and propane leaving their respective superfractionators (i.e., C.sub.2 and C.sub.3 splitters) as bottom products are recycled and cracked to extinction in the pyrolysis furnaces. Back-end acetylene hydrogenation reactors are eliminated.
The C4-plus fraction leaving the bottom of the heat pumped depropanizer is fed to a conventional debutanizer to produce a C4 mix as overhead product. The bottom product from the debutanizer is combined with the bottoms from the low pressure stripper in the compression train and sent to the pyrolysis gasoline hydrotreater.
External refrigeration for the ethylene recovery process of the parent application is supplied only by a propylene refrigeration compressor. No ethylene refrigeration is required by the ethylene recovery process.
Any solvent that is useful for absorbing hydrocarbons is suitable as the absorbent in the intercooled and reboiled demethanizing absorber. Such solvents include, but are not limited to, any of the solvents identified in earlier Mehra patents for use in all embodiments of the Mehra process.
The process of the parent application is equally as useful for treating refinery off-gases as it is for treating cracked gases because its versatility enables it to be readily adapted to the great variety of such refinery feeds.
However, the process of the parent application does not provide a means for isolating and recovering even a portion of the large quantities of hydrogen which are typically present in thermally or catalytically cracked gases and in refinery offgases. All of the hydrogen in such gases is discharged as a part of its fuel gas product. There is accordingly a need for a method and a means for recovering this hydrogen.
U.S. Pat. No. 2,938,934 of R. B. Williams describes a process for recovery of ethylene which comprises successively compressing, cooling, and fractionating a gas mixture to remove C.sub.4+ hydrocarbons as a first bottom stream and then C.sub.2 /C.sub.3 hydrocarbons as a second bottom stream which is fractionated to send C.sub.3 hydrocarbons to storage and to form an overhead stream which is hydrogenated and fed to a C.sub.2 splitter, the second overhead being cooled and partially liquefied to isolate a stream of hydrogen and methane which is fed to a methane absorber utilizing ethane as the solvent for methane, thereby isolating hydrogen for the hydrogenation operation.
U.S. Pat. No. 4,654,063 of Auvil et al discloses a process for recovering up to about 98% pure hydrogen from a diversity of hydrogen-containing gas streams by feeding such a gas mixture to a suitable non-membrane separation unit for treatment and separation to produce a hydrogen-enriched stream and a hydrogen-depleted stream. The non-membrane separation unit can be an adsorption, absorption, cooling, or partial condensation and/or rectification type unit. At least a portion of the hydrogen-enriched stream is fed to a membrane unit in which it is separated to form a hydrogen-rich permeate stream and a hydrogen-lean reject stream which is recycled to the nonmembrane separation unit.
U.S. Pat. No. 4,740,222 of Y. R. Mehra discloses a process for countercurrently extracting a hydrogen-containing inlet gas stream with a lean solvent stream to produce an overhead stream of at least 90% purity hydrogen and a bottom stream of rich solvent which is flashed in one or more stages. The flashed gas from at least the first stage, after compression, is recycled to the extracting step. The flashed gases from the remaining stages are recovered as fuel gas. When a hydrogen purity of more than 95% is needed, a minor portion of the stripped solvent from the last flashing stage is regenerated in a distillation column to form very lean solvent which is fed to the top of the extractor column while the major portion of the stripped solvent is fed to its middle.
U.S. Pat. No. 4,743,282 of Y. R. Mehra describes a process for treating cracked gases which have been compressed, cooled, sweetened, and dried to produce a C.sub.2 =+ hydrocarbons product, a methane-rich gas product, and a H.sub.2 -rich gas product by successive countercurrent extraction with two lean solvents in separate loops.
U.S. Pat. No. 4,832,718 of Y. R. Mehra teaches a method for hydrogen purification by countercurrently and successively extracting an olefins containing gas stream, at a pressure no greater than 500 psi, in an ethylene extractor column with a solvent slip stream from at least one flashing stage and then with lean solvent in a methane extractor column.
U.S. Pat. No. 5,019,143 of Y. R. Mehra describes a continuous process for contacting a hydrogen off-gas stream, at any pressure, in a demethanizing-absorber column, having at least one reboiler, with a main stream of stripped physical solvent and then with a cleanup stream of lean solvent.
The following patents of Yuv R. Mehra U.S. Pat. Nos. 4,421,535; 4,511,381; 4,526,594; 4,578,094; 4,601,738; 4,617,038; 4,623,371; 4,692,179; 4,680,042; 4,696,688; 4,740,222; 4,743,282; 4,832,718; 4,883,514 and 5,019,143 are incorporated herein by reference.